1. Field of the Invention
The present invention relates to a motor control apparatus, in particular, to a vehicle-steering control apparatus such as an electric power steering apparatus and an automatic steering apparatus for automobiles.
2. Description of the Prior Art
Heretofore, it has been commonly conducted to increase power supply voltages in order to raise an efficiency of large-power vehicular equipment. Although power supply (battery) voltages in passenger cars, for example, has generally been set at 12V, power supply voltages in commercial cars such as trucks has been set at 24V as they need a greater power supply. It has also been considered to set power supply voltages at 42V even in passenger cars as the number of equipment mounted therein increases. Moreover, in electric cars, a power supply of high voltages as much as 200V or 300V is even mounted.
Whereas, a microcontroller or an operational amplifier implemented in a control device for controlling a variety of vehicular equipment has been generally used at low voltage such as 3.3V, 5V, and 8V, etc. Applying high power supply voltage to such a control device causes a greater voltage drop in a power supply circuit, resulting in greater loss in the circuit.
There has been a known electric power steering apparatus, as a motor control apparatus to be mounted in a high voltage car system described above, as disclosed in Japanese Patent Laid-open Publication JP 2002-166837 (U.S. Pat. No. 6,600,983). In the above electric power steering apparatus, as shown in FIG. 3, a battery 1 supplies a high voltage power directly to a driving means 32 for a motor 2 and also supplies a lower voltage stepped down by a switching regulator 4 to a control unit 31 including a microcontroller and others through a series regulator 33, thereby a steep voltage drop at the series regulator 33 can be suppressed, even in a vehicle equipped the battery 1 with relatively higher voltage such as 24V,36V, preventing a power loss in the power supply circuit of the control unit 31.
Whereas, the driving means 32 is directly power-supplied from the battery 1, therefore the driving means 32 can be driven with higher efficiency to realize the control device with higher efficiency, as a whole.
However, there has been the problem in the aforementioned electric power steering apparatus that a power consumption in the battery becomes larger because the switching regulator 4 has been always operated, notwithstanding the control unit 31 comprising the microcontroller and others is operated or not, supplying electric power continuously to the series regulator 33. Once the switching regulator 4 started its self-excited oscillation, for example, its oscillation sustains even after switching off of an ignition switch, resulting in power consumption in the battery 1 even during an off-time of the ignition switch.